Protective circuit



A May 31, 1938. R w -r 2,119,372

PROTECTIVE CIRCUIT Filed Dec. 29, 19:56

film"- I 110 v.

' 3nnentor Karl R.Wendz Patented May 31, 1938 UNITED STATES PATENT OFFICE PROTECTIVE CIRCUIT aware Application December 29, 1936, Serial No. 118,054

7 Claims.

My invention relates to protective circuits and particularly to protective circuits for electronic tubes such as cathode ray tubes.

In electrical circuits it is often desirable either to delay the application of a high positive voltage to certain electrodes of a cathode ray tube until after the cathodes of certain vacuum tubes have become heated to an electron emissive state or to maintain the control grid of a cathode'ray tube so highly negative that the tube is blocked until the cathode becomes electron emissive. The reason for such precautions is to prevent injury to the cathode ray tube fluorescent screen caused by the cathode ray remaining stationary on the screen before the deflecting circuits become operative.

An object of my invention is to provide an improved protective circuit of the general character described above.

A further object of my invention is to provide an improved circuit for maintaining the control grid of a cathode ray tube highly negative until a deflecting field is applied to the cathode ray.

A further object of my invention is to provide an improved protective circuit in which a protective tube functions also as the rectifier of a voltage supply unit.

In a preferred embodiment of my invention the control grid of a cathode ray tube is biased negatively a large amount as soon as power is applied to the cathode ray tube and associated circuit. The protective circuit includes a rectifier tube having a thermionic cathode which heats more slowly than the cathodes of the tubes in the deflecting circuits. This rectifier tube, which supplies biasing voltages to various tube electrodes, has a relay winding in series therewith which operates certain contacts as soon as the cathode of the protective rectifier tube becomes 40 electron emissive. This causes one pair of contacts to short out a resistor applying the high negative bias to the cathode ray tube grid and causes another pair of contacts to close a circuit which supplies normal bias to the cathode ray 45 tube grid.

Other objects, features and advantages of my invention will appear from the following description, taken in connection with the accompanying drawing in which the single figure is a circuit dia- 50 gram of one embodiment of my invention.

Referring to the drawing, my invention is shown applied to a television receiver comprising a cathode ray tube l and associated circuits. The cathode ray tube may be of conventional de- 55 sign including an indirectly heated cathode 2, a

control grid 3, a first anode 4, a second anode 6 and a fluorescent screen I, the cathode ray being electrostatically focused. Suitable deflecting devices such as coils 5 and Ill are provided.

The second anode 6 is supplied with a high voltage from a suitable rectifier-filter unit 8.

The first anode 4 is supplied with a suitable positive voltage from another rectifier-filter unit 9 having resistors H and I2 connected thereacross which may have the resistance values in- 10 dicated on the drawing or some other suitable values. A voltage divider resistor l3 may be provided, if desired, as sometimes is desirable for supplying voltages to other tube electrodes such as a screen grid of a cathode ray tube when a 15 tube of this type is employed.

It will be noted that the cathode 2 is connected to the junction point of the two resistors II and I2 whereby, in the specific circuit shown, fourfifths of the voltage supplied from the rectifierfilter unit 9 is applied to the first anode 4 when the resistor I2 is included in a circuit. The resistor I2 has a pair of relay contact points I shunted thereacross, whereby it is shorted out when the contacts I are closed. The contact points I may be shunted by a resistor I 4 and condenser Hi to reduce sparking. As will be explained later, the voltage drop across the resistor I2 is applied to the control grid 3 to bias it highly negative until the contact points I are closed.

Video signals, which include picture signals and synchronizing impulses, are applied to the control grid 3 of the cathode ray tube I through amplifier tubes I5 and I1, which may be of the screen grid type. The tube H has a cathode iii, a control grid IS, a screen grid 2| and a plate 22. A suitable potential is applied to the plate 22 through a plate resistor 23 and the plate 22 is connected directly through a conductor 24 to the control grid 3 of the cathode ray tube I.

The grid circuit of the amplifier tube I! includes a grid condenser 26 and a grid resistor 21 having such capacity and resistance values that the incoming positive synchronizing impulses, which are greater in amplitude than the picture signals, cause the control grid l9 to be suitably biased to make the amplifier tube I1 function as an undistorting amplifier for the picture signals. Also, since the bias on the grid l9 varies with the amplitude of the synchronizing impulses, the voltage drop across the plate resistor 23, due to the direct current component, varies accordingly, whereby the bias on the control grid 3 of the cathode ray tube is varied. This variation in the cathode ray tube grid bias is such as to give automatic background control, as described and claimed in application Serial No. 718,192, filed March 30, 1934, in the name of Waldemar J. Pooh, and assigned to the Radio Corporation of America.

In the above description, it has been assumed that there is a connection from the lower end of the plate resistor 23 to the cathode 2 of the cathode ray tube. This connection will now be described. It includes the voltage divider 28 of a rectifier-filter unit 29, this unit including COD-9 tact points II in series with the rectifier 3| of the unit whereby voltage appears across the voltage divider 28 only after the contact points II have been closed. A resistor 32 and acondenser 33 may be connected across the contact points II to reduce sparking. The connection between the cathode 2 and the lower end of the plate resistor 23 is completed by a connection fromthe negative end of the resistor |2 toa variable tap 34' on the voltage divider 28. It will be seen that the variable tap 34 may be adjusted to include in the grid cathode circuit of the cathode ray tube a voltage which is in opposition to the voltage drop across the plate resistor 23, whereby the bias on the control grid 3 may be adjusted to any desired value for a given incoming signal. This adjustment is made when the contact points I are closed, this being their position during operation of the receiver. When the contact points I are open an additional negative voltage is applied to the control grid, this additional voltage being the voltage drop across the resistor |2.

A relay coil 36 for actuating the contact points I and II is included in the circuit of a rectifier tube 31 having an anode 38, a cathode 39, and a tungsten filament heater 4|. The filament 4| is connected to a suitable current source through a. resistor 42. Since the filament 4| has a low cold resistance, a comparatively small voltage is applied across it initially, this voltage increasing in value as the filament heats up and as its resistance becomes greater. In this way, the oathode 39 of the rectifier tube 31 is caused to become electron emissive at a later time than the cathodes of the other tubes in the receiver. It is assumed that power is supplied to the cathodes of filaments of all tubes simultaneously.

The cathode 39 and the plate 38 of the rectifier tube 31 are connected across a suitable voltage source and in such direction as to provide biasing voltages for the various tubes in the re-- ceiver. In the specific circuit illustrated, the cathode 39 is connected to a plate terminal of the high-voltage winding 44 in the rectifier-filter unit 29. The plate 38 is connected through a resistor 46, the relay coil 36, a second resistor 41 and a voltage divider 48 to the center point on the high-voltage winding 44. In order to maintain the voltage across the voltage divider 48 substantially constant, filter condensers 49 and a voltage regulating glow tube 5| are included in the rectifier circuit in shunt to the voltage divider.

The operation oithe circuit is as follows: With no power applied to the receiver, there is no current flowing through the relay coil 36 and the contact points I and II are open. As soon as power is applied to the receiver, the cathodes of all the tubes begin to heat up and, in a short time, all these cathodes are electron emissive except the cathode of the protective rectifier tube 31. Also it should be noted that the filaments of the rectifier tubes in the anode supply units 8 and 9 are directly heated and will become electron emissive at almost the instant the power is supplied to the receiver and considerably before the cathodes 2 and I8 and the cathodes of the tubes in the deflecting circuits (not shown) become electron emissive.

Thus, it will be seen that after power has been applied to the receiver and before current begins to flow through the relay coil 36, there is no current flowing through the voltage divider 28, as the contact points 11 are open, and, since the contact points I are also open, the voltage drop across the resistor I2 is applied to the control grid 3; of the cathode ray tube to bias the tube considerably beyond its cut-off point.

As soon as the cathode of the protective tube 31 becomes heated to an electron emissive state, current flows through the relay coil 36. By this time the cathodes of the various receiver tubes, including those in the deflecting circuits, have become heated to their proper operating temperature and deflecting current has been supplied to coils 5 and H). V The current flowing through the relay coil 36 closes the contact points I and II. The closingof contact points I removes the protective biasfrom the grid 3 of the cathode ray tube and applies full voltage to the first anode 4. The simultaneous closing of the contact points II causes current to flow through the voltage divider 28 and applies a potential to the plate 22 of the amplifier tube Thus, the protective bias is removed and normal bias is applied only after a deflecting field is supplied. The change from protective'bia's to normal bias either takes place simultaneously or the contact points are so adjusted that the protective bias is removed an instant after contact points II have closed to apply normal bias to the control grid.

It will be seen that the protective rectifier tube 31 has a double function, since it acts both as a delay tube for operating the relay contact points I and II and as a rectifier tube for supplying biasing voltages to the various receiver tubes.

It may be pointed out that the tubes in the deflecting circuits (not shown) may be supplied with voltage from unit 29, in which case the deflecting current is supplied to the coils 5 and III the instant contact points II close.

In addition to protecting the cathode ray tube, my circuit protects the mercury rectifier tube 3| and electrolytic condensers such as the filtercondenser 43 in the plate circuit of tube I5, since it gives the cathodes of tubes 3| and I5 time to become electron emissive before voltages are applied to their plates.

In the drawing, certain voltage and resistance values and oer-tain tube types have been indicated merely by way of example.

I claim as my invention:

1. In combination, a cathode ray tube having a cathode, a control grid and an anode, a voltage supply unit having two resistor elements con nected thereacross, said anode being connected to the positive terminal of said unit and said cathode being connected to the junction point of said resistor elements whereby one of said elements is connected between said cathode and said anode, an amplifier tube having a plate resistor, a direct current connection between the nection between the negative end of the other of said resistor elements and a point on said voltage divider whereby the voltage drop across said other resistor element is applied to said control grid, and means for short-circuiting said other resistor element.

2. The invention according to claim 1 characterized in that said last means includes a relay comprising a relay coil and contact points, said contact points being connected across said other resistor element, said relay coil being connected in series with a thermionic device which passes current to actuate said relay only after a predetermined time.

3. Electrical apparatus comprising a cathode ray tube and a plurality of electric discharge tubes, said cathode ray tube having a control grid, means for applying a high negative bias to said control grid when power is first supplied to said apparatus, and means for removing said bias after the power has been supplied to said apparatus for a predetermined time, said last means including a rectifier tube with a slow heating cathode and having a relay coil in series therewith, a voltage divider connected to be supplied from said rectifier tube, and filter elements connected in shunt to said voltage divider whereby a substantially steady voltage may be obtained therefrom.

4. Electrical apparatus comprising an amplifier tube having a plate, a cathode ray tube having a cathode and a control grid, a power supply unit having a rectifier and a voltage divider, said plate being connected to said voltage divider through a plate resistor, a direct current connection from said cathode to said control grid, said connection including said voltage divider and said plate resistor, and means for connecting said voltage divider in series with said rectifier in response to power being supplied to said receiver for a predetermined time.

5. The invention according to claim 4 characterized in that said last means includes the rectifier tube of a power supply unit and a relay coil connected in series with said rectifier tube.

6. Electrical apparatus comprising an amplifier tube having a plate, a cathode ray tube having a cathode, a control grid and an anode, a power supply unit having two resistor sections connected in series with each other and across said unit, one of said sections being connected between said cathode and said anode, a second power supply unit having a rectifier and a voltage divider, said plate being connected to said voltage divider through a plate resistor, a direct current connection from said cathode to said control grid, said connection including the other of said resistor sections, said voltage divider, and said plate resistor in series, means for short-circuiting said other resistor section in response to power being supplied to said apparatus for a predetermined time and means also for connecting said voltage divider in series with said rectifier in response to power being supplied to said apparatus for a predetermined time.

'7. The invention according to claim 6 characterized in that said last two means include a relay coil connected in series with a rectifier tube having a slow heating cathode.

KARL R. WENDT. 

